xen/kbdif: add multi-touch support

Multi-touch fields re-use the page that is used by the other features
which means that you can interleave multi-touch, motion, and key
events.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>